Efficient operation of a waste fluid plant, such as for sewage treatment, requires control over the fluid flow rate over a wide range of flow volumes. The flow volumes typically vary by about 3:1 between peak and minimum flows. Further, the overall flow through a plant may expand considerably over the operating life of a plant, because of, for example, growth in the areas served by the plant. Good design requires that a plant be able to accommodate fluid levels of perhaps two to three times initial anticipated peak and minimum flows.
Fluid control is particularly important in relation to screening equipment, such as a bar rack composed of a plurality of spaced bars that acts as an initial screening device to trap and remove relatively large debris from the waste fluid inlet channel. If the flow rate is too high (greater than about 3.28 feet per second), the debris that desirably is trapped by the rack aligns with the fluid stream and is forced through the rack, thereby fouling subsequent treating apparatus and causing environmental and other problems.
Conversely, the fluid flow should not be too low (less than about 1.25 feet per second). This is because the fluid often contains smaller waste, generally termed "grit," composed of material such as sand, gravel, small rocks, degraded concrete and tile sewer pipe. This grit is too small to be intercepted by the rack, and is intended to pass therethrough for subsequent removal. If the fluid velocity becomes too low, the grit will settle within the channel, smell badly, interfere with the operation of the bar screen and, over time, reduce the volumetric capacity of the channel to such an extent that some form of cleaning is required.
For the above reasons, it is desirable to control the velocity of fluid through a waste water treatment plant, and particularly at the location of screen apparatus. The initial design choice of channel width and depth is not easily changeable, once the channel has been constructed. Downstream regulating devices, such as, for example Parshall flumes, Sutro weirs, and Camp flow regulators, may be used to control the depth of water flow and thus its velocity. Other devices are known that in some way control water flow. For example, U.S. Pat. No. 3,926,805 of Walker describes a gate inlet system with a tangential gate that imparts a rotational flow to water of a waste treatment plant. U.S. Pat. No. 4,167,358 of Besha describes an inflatable bladder that may be selectively deployed to block an open culvert. U.S. Pat. No. 4,300,858 of Zintz et al. describes a variable dam including a plurality of plates that may be rotated into or out of a stream path. U.S. Pat. No. 5,378,376 of Zenner describes a sludge collector that has an adjustable weir to match the rate of flow into the collector and rate of flow out of the collector.
None of the known prior art devices and methods provide a simple method to vary flow rates over both short and long term fluctuations, with a minimum of moving parts. Further, none of the known prior art devices are specifically adapted for use with a bar rack; the prior art devices may thus interfere with the cleaning operation used in conjunction with bar racks.
The present invention is in some ways similar to the invention described in U.S. patent application Ser. No. 08/735,525, issue fee paid Apr. 29, 1998, the contents of which are incorporated by reference. The present invention describes alternative and additional structural arrangements to accomplish broadly the objectives of the Ser. No. 08/735,535 application.
It should be understood that nothing herein is admitted to be prior art, but is only mentioned to help place the present invention in context.